Chemists Find Material To Both Generate And Store Hydrogen Fuel

Imagine a device that can not only store energy but can also produce it. Now put your imagination aside and consider a very real substance, the semiconductor molybdenum disulfide. Chemists at Rice University in Houston say it can accomplish both tasks inexpensively and efficiently.

From one angle, molybdenum disulfide (MoS2), with its honeycomb lattice structure, looks like graphene, a sheet of carbon one-atom thick. But seen edge-on with a powerful microscope it shows that it’s actually made up of three distinct layers: a layer of sulfur atoms trapped between two layers of molybdenum.

Because MoS2 is three-ply, though, its edge is better suited not just for energy storage, but also for catalytic reactions for generating hydrogen energy. This may sound like a particularly insubstantial part of any material in sheet form. After all, how much can you write on the edge of a piece of paper while ignoring its surface?

But chemist James Tour, who led the research team at Rice, says there’s more on the edge of molybdenum disulfide than meets the eye.

“So much of chemistry occurs at the edges of materials,” Tour told the university’s News & Media department. “A two-dimensional material is like a sheet of paper: a large plain with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.”

Making molybdenum disulfide was less than complicated. Tour’s team simply coated a layer of plain molybdenum with a film of molybdenum oxide using room-temperature anodization, the same method used to protect some metals from corrosion. Then they exposed the coated molybdenum to sulfur vapor at 300 degrees Celsius (572 degrees Fahrenheit) for one hour.

They put the resulting molybdenum disulfide through several tests, including determining whether it could be used as a supercapacitor, a rechargeable battery with a long duty cycle. What they found was that a MoS2 supercapacitor kept 90 percent of its capacity after 10,000 recharges and 83 percent capacity after 20,000 recharging cycles.

This means there are potentially many uses for the technology. “We see anodization as a route to materials for multiple platforms in the next generation of alternative energy devices,” Tour said. “These could be fuel cells, supercapacitors and batteries. And we’ve demonstrated two of those three are possible with this new material.”

Molybdenum disulfide also can be used to produce hydrogen. So far isolating the gas has been problematic and expensive, but alternative methods are emerging, including using solar energy to extract hydrogen from water, a process known as a hydrogen evolution reaction, or HER.

The process developed at Rice is similar. Until now, the solar energy method required costly platinum, but molybdenum disulfide, which is inexpensive to make, does the job just as well.